This invention relates to a compressor which has discharge valve means adapted to enhance the coefficient of performance of the compressor.
In a compressor in general, which compresses fluid, a cylinder accommodating a piston or a rotor has an outer surface formed thereon with at least one flat valve seating surface at a predetermined location and also formed therein with outlet holes communicating with a pump working chamber defined between an inner surface of the cylinder and the piston or the rotor, and a discharge valve is arranged over the valve seating surface to close and open the outlet holes. The minimum possible thickness of the outer wall of the cylinder should be more than a value corresponding to required yield strength of the compressor. Accordingly, also a portion of the cylinder wall formed with the above valve seating surface has to have a sufficient thickness at least equal to the minimum possible thickness. However, if the above portion of the cylinder wall has such a sufficient thickness, the internal spaces of the outlet holes have a considerably large overall volume, as is the case with a conventional compressor. This large internal volume of the outlet holes forms a great part of the dead volume of the compression space of the compressor, i.e. a great part of the clearance volume of the pump working chamber formed at the termination of the compression stroke, resulting in degraded coefficient of performance of the compressor.
This is also the case with a conventional vane compressor which has a cylinder having an endless camming inner peripheral surface, and a rotor rotatably received within the cylinder and carrying vanes slidably fitted therein, wherein pump working chambers are defined between the endless camming inner peripheral surface, the outer peripheral surface of the rotor, and adjacent ones of the vanes. That is, the aforementioned problem of degraded coefficient of performance is caused by a series of outlet holes with a large internal volume, formed through the peripheral wall of the cylinder and communicating with a pump working chamber on the compression stroke. To be specific, when tip of a preceding vane passes the outlet holes opening in the camming inner peripheral surface of the cylinder, residual compressed gas in the internal spaces of the outlet holes leaks into a pump working chamber on the compression stroke defined by the succeeding vane. Consequently, in the pump working chamber on the compression stroke, the internal pressure increases, resulting in an increase in the power required for the compression and accordingly increased energy loss of the compressor. Also, the leaked compressed gas is again compressed, causing an increase in the temperature of the discharge gas.
To improve the compression efficiency of the compressor, it is therefore required to reduce the internal volumes of the outlet holes which form a great part of the dead volume of the compressor.
To meet the above requirement, there has been proposed a discharge valve for refrigerant compressors, as shown in FIG. 1, by U.S. Ser. No. 783,834 filed Oct. 3, 1985 (now U.S. Pat. No. 4,668,172, issued May 26, 1987) assigned to the assignee of the present application, which is a continuation-in-part application of abandoned U.S. application No. 575,889 filed Feb. 1, 1984 (by Japanese Utility Model Registration Application No. 61-019651 filed Feb. 14, 1986 in Japan), which comprises a plurality of arcuately concavely curved grooves 14 circumferentially formed in an outer peripheral surface of the cam ring 2a of the cylinder 2 at locations where outlet holes 10 are radially formed through the cam ring 2a so as to minimize the wall thickness of the above locations, a plurality of annular valve bodies 11 formed by an elastic sheet and received in the respective grooves 14 for urging contact with bottom surfaces 14a of the respective grooves 14 for opening and closing the respective outlet holes 11, a holder member 12 mounted on the outer peripheral surface of the cam ring 2a and supporting the valve bodies 11, and a plurality of stoppers 13 secured to the holder member 12 and located within the respective valve bodies 11 for preventing excessive deformation of the valve bodies 11 away from the outlet holes 10.
With such construction of the discharge valve, the outlet holes 10 can be greatly reduced in length by forming the arcuate grooves 14 in the outer peripheral surface of the cam ring 2a at locations where the outlet holes 10 are radially formed therethrough, so as to minimize the wall thickness of the above locations, thus minimizing the dead volume formed by the outlet holes 10.
However, according to the discharge valve as constructed as above, in forming the grooves 14 by means of a cutter such as a milling cutter, the entire bottom surfaces 14a of the grooves 14 have to be smoothly finished although just small areas of the bottom surfaces 14a are in contact with the valve body 11. In addition, ribs 14b are formed between the adjacent grooves 14 so as to reinforce the strength of the peripheral wall of the cam ring 2a, but they interfere with the rotary shaft of the milling cutter inserted into the grooves 14, which makes it impossible to use a small-sized milling cutter capable of forming grooves with the same radius of curvature as that of the valve bodies 11.
However, the use of a large-sized milling cutter which does not interfere with the ribs 14b makes it impossible to machine the bottom surfaces 14a of the grooves 14 to such a small radius of curvature as is equal to that of the valve bodies 11. Consequently, the radius R of curvature of the bottom surfaces 14a is too large, as compared with the radius r of curvature of the valve bodies 11, to ensure tight contact of the valve bodies with the seating surface portions of the bottom surface 14a.
If the valve bodies 11 are arranged to be strongly forced against the seating surface portions in order to obtain tight contact therebetween, the valve bodies cannot be opened by a suitable pressure force of the compressed gas, resulting in excessive compression of the gas and degraded performance of the compressor.